1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copier, a printer, and a facsimile, and particularly to a wet-type image forming apparatus using liquid developer to form a toner image.
2. Description of the Related Art
In an electrophotographic image forming apparatus, toner supplied from a developing device is used to develop an electrostatic image on a photoconductor. Then, the developed toner image is transferred onto a recording paper, whereby an image is formed on the recording paper. In the transfer process in such an image forming apparatus, an electrostatic transfer technique is generally adopted.
When a toner image is transferred onto recording paper serving as a transfer target, voltage is applied by a transfer roller from the back side of the recording paper conveyed to a position opposing the photoconductor to form an electric field between the photoconductor and the recording paper. The electric field causes the toner image to be electrostatically adsorbed on the recording paper.
Thereafter, the toner image formed on the recording paper is heated and pressed by a fixing device, whereby the transferred toner image is fixed on the recording paper.
On the other hand, in recent years, in an image forming apparatus such as an office printer for bulk printing and an on-demand printer that requires higher image quality and higher resolution, a wet-type developing device is known, which uses a liquid developer with a small toner particle size that is less likely to produce toner image noise. The liquid developer is prepared by dispersing toner in a carrier liquid such as a paraffin-based solvent. In the development and transfer process, toner is moved by the effect of an electric field in a toner layer containing the carrier liquid and the toner, whereby an image is transferred onto recording paper.
In order to obtain high-quality images in the wet-type developing device, it is necessary to transfer images at an appropriate image density. In this case, the image density is determined by the amount of toner on the developing roller. Therefore, it is important to stably control a thin layer of liquid developer (toner layer thickness) on the developing roller.
The liquid developer thin layer (toner layer thickness) varies with a change in viscosity of the liquid developer due to a temperature change, an error of the apparatus, and the like. Thus, it becomes necessary to sense and control the amount of the liquid developer thin layer.
Japanese Laid-Open Patent Publication No. 2010-014892 discloses a method of sensing a surface potential of a developing roller and controlling the amount of a liquid developer thin layer such that the surface potential is constant.
Here, the toner charged by a charger has a potential as a result of charge, and its surface potential (toner layer potential) is determined by the toner charge amount and the amount of adhered toner. Therefore, when the toner charge amount is constant, the amount of adhered toner on the developing roller can be found by measuring the surface potential (toner layer potential).
In order to keep the toner charge amount constant, the flow-in current from the charger for charging should be kept constant. In order to do so, it is necessary to measure the current flowing into the developing roller from the charger and to control the current at a constant value.
However, in actuality, not only the current from the charger but also the current flowing between the developing roller and the photoconductor and the like flows into the developing roller. Therefore, it is difficult to measure only the flow-in current from the charger.
There is another problem that even when current flowing from the power supply to the charger is controlled to be constant, stains on the charger, an environmental change, and the like change the current flowing into the developing roller.
If the surface potential (toner layer potential) is measured in this state and the amount of adhered toner is calculated, the measured amount of toner adhesion differs from the actual amount, because the toner charge amount is also changed due to variations of the flow-in current. Therefore, the amount of the liquid developer thin layer cannot be controlled accurately.